In recent years, in the field of electronic devices, the higher densities and higher integration degrees of electronic circuits have led to a demand for further reductions in size and improvements in the performance of capacitor element as an essential circuit element in a variety of electronic circuits.
For example, thin film capacitors using single layer of dielectric thin film have been slow in being reduced in size in integrated circuits with transistors and other active elements and have become factors obstructing the realization of ultra-high integrated circuits. The delay in reducing the size of thin film capacitors is due to the low permittivity of the dielectric materials used for them. Therefore, to reduce the size of thin film capacitors and realize higher capacitances, use of dielectric materials with higher permittivity is important.
Further, in recent years, from the viewpoint of the capacitance density, the conventional SiO2 and Si3N4 multilayer films are no longer sufficient for the capacitor materials for the next generation DRAMs (Gigabit generation), and material systems with higher permittivity have gathered attention. Among such material systems, use of TaOx (∈=up to 30) has mainly been studied, but other materials are also being actively developed.
On the other hand, as dielectric materials with relatively high permittivity, (Ba,Sr)TiO3 (BST) and Pb(Mg1/3Nb2/3)O3 (PMN) are known.
Therefore, if using this type of dielectric material to form a thin film capacitor element, it should probably be possible to reduce the size.
However, when using this type of dielectric material, the reduction in thickness of the dielectric film leads to a drop in the permittivity. Further, reduction in thickness leads to the formation of holes in the dielectric film and thereby deterioration of the leakage characteristic and withstand voltage. Further, the formed dielectric film tends to be poor in surface flatness and poor in rate of change of the permittivity with respect to temperature. Note that in recent years, due to the environmental impact of PMN and another lead compounds, lead-free high capacitance capacitors have been desired.
As opposed to this, to realize a reduction in size and increase in capacitance of multilayer ceramic capacitors, it has been desired to reduce the thickness of each of the dielectric layers as much as possible and to increase the number of the dielectric layers in a predetermined size as much as possible.
However, for example, when using the sheet method (method of using a dielectric layer paste to form a dielectric green sheet layer on a carrier film by the doctor blade method etc., printing an internal electrode layer paste over this in a predetermined pattern, then peeling off and stacking one layer at a time) to produce a multilayer ceramic capacitor, the dielectric layers cannot be formed thinner than the ceramic material powder. Further, due to short-circuits caused by defects in the dielectric layers, breakage of the internal electrodes, or other problems, it has been difficult to reduce the dielectric layers in thickness to for example 2 μm or less. Further, when reducing the thickness of each of the dielectric layer, there were also limits to the number of layers. Note that there are similar problems when using the printing method (for example, the method of using the screen printing method to alternately print dielectric layer paste and internal electrode layer paste on a carrier film a plurality of times, then peeling off the carrier film) to produce a multilayer ceramic capacitor.
Due to such a reason, there were limits to the reduction of size and improvement of capacitance of multilayer ceramic capacitors. Therefore, various proposals have been made to solve this problem (for example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5, etc.)
These publications disclose methods of production of multilayer ceramic capacitors by using CVD, vapor deposition, sputtering, or other various types of thin film forming methods to alternately stack dielectric thin films and electrode thin films.
However, the dielectric thin films formed by the methods described in these publications are poor in surface flatness and, when overly stacked, suffer from electrode short-circuiting. Due to this, it was only possible to produce devices with at the most 12 to 13 layers. For this reason, even if the capacitor could be reduced in size, a higher capacitance could not be achieved.
Note that as shown in Non-Patent Document 1, the fact that a composition of the formula (Bi2O2)2+(Am−1BmO3m+1)2− or Bi2Am−1BmO3m+3 where, in the formula, the symbol m indicates a positive integer of 1 to 8, the symbol A indicates at least one element selected from Na, K, Pb, Ba, Sr, Ca, and Bi, and the symbol B indicates at least one element selected from Fe, Co, Cr, Ga, Ti, Nb, Ta, Sb, V, Mo, and W forms a bulk bismuth layered compound dielectric obtained by the sintering method is itself known.
However, this publication did not disclose anything at all regarding under what conditions (for example, the relationship between the surface of the substrate and the c-axis orientation of the compound) the thickness is reduced (for example 1 μm or less) is it possible to obtain a relatively high permittivity and low loss even when thin and whether a thin film superior in leakage characteristic, improved in withstand voltage, superior in temperature characteristic of the permittivity, and superior in surface flatness can be obtained.
Therefore the inventors first developed and filed for thin film capacitor element compositions shown in the following Patent Document 6 and Patent Document 7. The inventors engaged in further experiments and as a result discovered that by including a predetermined amount of a rare earth element in a bismuth layered compound and lowering the Curie point, the frequency characteristic, bias characteristic, and dielectric loss can be improved and thereby completed the present invention.
Note that in a bismuth layered compound dielectric material, addition of lanthanum or another rare earth element has been a conventional practice. For example, use of MOCVD (abbreviation for metal-organic vapor deposition) or single crystal growth etc. to introduce a predetermined amount of lanthanum into a bismuth layered compound is shown in Non-Patent Document 2 and Non-Patent Document 3. Further, Non-Patent Document 4 shows the relationship between the lanthanum content and Curie point in a bismuth layered compound dielectric material.
The content of lanthanum in the bismuth layered compound dielectric described in these publications was, in Non-Patent Document 2, x=0.2 to 0.9 in range in Bi4−xLaxTi3O12, in Non-Patent Document 3, x=0.25 to 0.75 in range in Bi4−xLaxTi3O12, and, in Non-Patent Document 4, x=0.10 to 1.00 in range in SrBi4−xLaxTi4O15 and x=0.05 to 0.75 in range in Sr2Bi4−xLaxTi5O18. Further, with the methods of production of the bismuth layered compound dielectric materials described in these publications (for example, MOCVD and single crystal growth), introduction of lanthanum (rare earth element) in a content of more than that described in the publications is difficult.
Patent Document 1: Japanese Patent Publication (A) No. 2000-124056
Patent Document 2: Japanese Patent Publication (A) No. 11-214245
Patent Document 3: Japanese Patent Publication (A) No. 56-144523
Patent Document 4: Japanese Patent Publication (A) No. 5-335173
Patent Document 5: Japanese Patent Publication (A) No. 5-335174
Patent Document 6: PCT/JP02/08574
Patent Document 7: Japanese Patent Application No. 2003-12086
Non-Patent Document 1: “Grain Orientation of Ferroelectric Ceramics having Bismuth Layered Structure and Application Thereof to Piezoelectric and Pyroelectric Materials”, Takenaka, Tadashi, Kyoto University Doctoral Thesis (1984), Chapter 3, pp. 23 to 77
Non-Patent Document 2: Takayuki Watanabe and three others, “Site definition and characterization of La-substituted Bi4Ti3O12 thin films prepared by metal organic chemical vapor deposition”, Journal of Applied Physics, Dec. 15, 2001, vol. 90, no. 12, p. 6533-6535
Non-Patent Document 3: Rintaro Aoyagi and three others, “Crystal Growth and Characterization of Lathanum Substituted Bismuth Titanate Single Crystal”, Japanese Journal of Applied Physics, September 2001, vol. 40, no. 9B, p. 5671-5674
Non-Patent Document 4: Jun Zhu and four others, “Study on Properties of Lathanum Doped SrBi4Ti3O15 Ferroelectric Ceramics”, Japanese Journal of Applied Physics, August 2003, vol. 42, no. 8, p. 5165-5168